1. Field of the Invention
This invention relates to photomultiplier tubes and in particular, to a microchannel plate photomultiplier tube that provides suppression of ions generated throughout the microchannel plate when the photomultiplier tube is in operation.
2. Description of the Related Art
During operation of a transmission-mode microchannel plate photomultiplier tube (MCP-PMT) positive ions are generated along the length of the MCP pores and are accelerated directly towards the photocathode, where they impact with significant energy. This phenomenon is termed “ion feedback” and is responsible to a significant degree for degradation of photocathode sensitivity and adversely affects the expected lifetime of the device. There are known techniques directed at reducing or eliminating the ion feedback effect that generally involve reducing the number of ions through the use of sophisticated materials engineering and/or vacuum processing. Alternatively, physical ion barriers formed in the MCP geometry and/or ion barrier films deposited on an external surface of the MCP have been used.
In a transmission-mode MCP-PMT, photons are detected by their absorption and the subsequent ejection of photoelectrons from a semi-transparent photocathode deposited on the vacuum side of a window. The photoelectrons are amplified by a factor of at least 103 by means of a secondary-electron cascade in one or more MCP's. The electrons emitted by the MCP are collected as charge pulses on a single or multi-segment anode. The operational principle of a PMT having a single MCP is illustrated in FIG. 1. An MCP-based image intensifier tube operates according to the same principle as the MCP-PMT, but the charge collecting anode is replaced by an imaging system.
MCP's are wafers containing millions of high aspect-ratio hollow channels, the walls of which have been treated to provide a desired electrical conductivity and a high probability of releasing secondary electrons. Generally, MCP's are made using leaded-glass, although the use of conformal thin-film coatings has more recently enabled MCP's to be fabricated using other substrate materials.
When an energetic primary particle such as a photoelectron strikes the wall of an MCP pore channel, it can release one or more secondary electrons. In MCP-PMTs this initial event is facilitated by (i) accelerating the photoelectron across a potential difference of at least 100 V and (ii) orienting the MCP pores at an angle relative to the wafer normal direction. The secondary electrons are accelerated down the length of the pore channel by a large electric field (˜106 V/m) until they strike the channel wall and liberate additional secondary electrons. This cascade process is repeated numerous times as illustrated in FIG. 2 and results in a pulse comprising at least 1000 electrons leaving the output side of the MCP. The output electrons are then accelerated to the charge collecting anode.
Throughout the amplification process positive ions are also generated by electron-molecule collisions. Given the ultrahigh vacuum (UHV) conditions inside the MCP-PMT, direct ionization of residual gases is relatively unimportant and the ion generation occurs predominately by electron stimulated desorption (ESD) from the surfaces of the MCP pore channels. Inside the MCP pores the electric field is axial, so the ions generated can be accelerated out of the MCP back toward and into the photocathode where they adversely affect the lifetime of the device. For a typical MCP the ion yield increases exponentially along the length of the MCP pores in direct correlation with the electron density and as a result, there is an increasing distribution of higher energy ions originating nearer the output side of the MCP as illustrated in FIG. 3. If one neglects the relatively small internal energies from the ESD process, the high-energy cutoff of this distribution occurs at the full potential energy difference between the MCP output and the photocathode which is typically greater than 1000 eV.
A common method of minimizing ion feedback is to treat the MCP surfaces such that fewer ions are created during the multiplication process. At a minimum this is done through the use of UHV techniques involving extreme cleanliness in the handling and processing environments and extended bake-outs of the MCP at elevated temperature. Additionally, extensive operation of MCP's under UHV conditions before their assembly into the PMT allows the ESD process to “scrub” the MCP surfaces which also decreases the ion feedback rate. In addition, techniques that involve either conformally depositing on the MCP a film with desirable properties to minimize damaging ion feedback or functionalizing the MCP entirely through the use of conformal coatings of desired materials have been demonstrated in the art.
Complementing the ion-minimizing methods, one solution is to physically interrupt the ions while they are in transit towards the photocathode. Certain devices such as Gen III image intensifiers make use of a thin barrier film deposited over the input of the MCP that can ensure that energetic ions cannot reach the photocathode. However, that technique is not without drawbacks in complexity and in certain aspects of performance. Another physical-barrier technique is to arrange multiple MCPs in series with their pore channel directions staggered, such that the majority of ions are guaranteed to collide with the MCP channel surfaces. The most common configurations are termed “chevron” and “Z-stack” when using two or three plates, respectively. A chevron arrangement of MCPs is shown in FIG. 4A and a Z-stack configuration is shown in FIG. 4B. In these staggered configurations the majority of ions generated deep in the MCP pores are forced to strike the upper plate where the channel wall changes their direction and the number of ions reaching the photocathode is greatly reduced although not entirely eliminated.
The PLANACON photon detector is a square-shaped, multi-anode MCP-PMT that is manufactured and sold by PHOTONIS USA Pennsylvania Inc., of Lancaster, Pa. The PLANACON photon detector is used for many photon detection applications where large detection areas are required. The unique format of the PLANACON detector makes it the largest detector areally of its type on the market and allows for many PLANACON detector units to be tiled together in order to form a larger image.